1. Field of the Invention
This invention relates to array antennas and is particularly concerned with providing time delay steering to array antenna elements.
2. Description of the Prior Art
An array antenna consists of a group of antenna elements spaced apart usually at regular intervals to form an array. By either controlling the relative timing or the phasing of the common signal at the transmitting elements, a beam is propagated that can be steered in space.
The most common means of steering a beam in an antenna array is to control the relative phase of the signal at the elements. For the case of a flat antenna array, if all the elements are operated in unison, the beam will be pointed in the boresight direction, which is the direction perpendicular to the plane of the array. If a linearly increasing phase shift is introduced across the face of the array, the beam will be deflected at some angle from the boresight direction, and the larger the phase shift, the larger will be the angle between the beam direction and the boresight direction. Such antenna systems, referred to as phase arrays, are employed in applications where it is required to steer the beam rapidly in space.
Controlling the relative phase of each of the antenna elements requires that each element contain a phase shifting device and that an electronic control system be used to control the phase of each of the elements. However, the wide scale use of phased arrays has been limited by the high cost of their complex circuitry. Furthermore, if the phase shifting circuit is adjusted to steer in a particular direction, this setting will only be valid for a particular frequency. Therefore, known phase shifting techniques impose a limit on the frequency range of operation.
Another technique that is used to steer the beam in an array antenna is to control the relative timing of the transmitted signal at the radar elements. If the signal at each of the radiating elements is emitted in unison, a wave front is formed that is parallel to the plane of the array. The signal beam is directed perpendicular to the wave front, therefore, when the signal is emitted from the antenna elements in unison, the beam is directed perpendicular to the plane of the array (the boresight direction). When the emission from the antenna elements is not in unison, but is varied in time along the array, the angle of the wave front relative to the plane of the array will change and the beam will be steered away from boresight. If, for example, the signal emission from any element to its nearest adjacent element is delayed a time t and each element is spaced a distance d apart, the steered angle .PHI. between the boresight direction and the beam direction is given by the formula sin .PHI.=t c/d, where c represents the velocity of light. True time delay techniques allow antenna arrays to operate over extremely wide frequency ranges as the delay techniques are frequency independent.
The use of fiber optics in communication systems is known. The optical fiber is usually used to connect two distant points so that communications can be conducted. A commercially available laser unit is used to generate an optical signal. The optical signal travels through the optical fiber to where it is converted to a microwave signal by an optical detector and a microwave amplifier, which are commercially available. Fiber optic media offer the advantage of exceptional bandwidth as compared with other traditional transmission means such as copper wire.
Optical techniques have been suggested to control array elements. Schemes have been proposed to use a selection of optical fibers with lengths arranged in a binary or quadratic sequence and to switch in a series string combination to achieve a desired timing. This would result in a very complex control scheme employing thousands of optical fibers and optical switches for even the simplest phased array.